Studies of growth factor receptors provide unique opportunities to develop specific anticancer therapy in human breast cancer. The EGF receptor family contains structurally related transmembrane proteins that play a role in the development and progression of cancer. Members of this family including erbB-2 and the recently identified erbB-3 and erbB-4 genes, are often overexpressed in various human cancer cells. A ligand, termed heregulin (HRG/NDF) was cloned and showed to possess an EGF-like domain which is sufficient for activation of p185(erbB-2). Recently, it was found that both erbB-3 and erbB-4 are receptors for HRG, and presence of one of these two receptors is required for the activation of erbB-2 by HRG. The fact that many mammary tumor cells preferentially co- overexpress erbB-2 or the EGFR with erbB-3 and/or erbB-4 receptors, and their extracellular accessibility makes these proteins excellent potential targets for specific antitumor therapy. Pseudomonas exotoxin (PE) is a bacterial toxin that inactivates protein synthesis by catalyzing ADP-ribosylation of elongation factor 2. A number of recombinant toxins have been developed by fusing ligand or single chain antibodies to truncated PE. These chimeric toxins are very cytotoxic to target cells, and have potent in vivo tumor inhibiting activity. The proposed studies are designed to develop chimeric toxins, composed of heregulin and PE as a novel therapeutic agent for breast cancer treatment. We propose to characterize the clinical relevance of erbB-4 in human breast cancer and explore the erbB-4 based in vivo antitumor activity. A murine hematopoietic cell line (designated 32D) will be utilized to study a correlations between the specific expression of erbB receptors and chimeric toxins induced killing. 32D cells are devoid of many receptors (e.g., EGF, PDGF, HRG, erbB-2, etc.) and are strictly dependent on interleukin-3 (IL-3) for proliferation and survival. This IL3 dependence can be abrogated by activated surrogate receptors. This approach should assist the studies of erbB receptors interaction and signal transduction pathway. This program is designed to ultimately generate specific, effective recombinant toxins with unique targets that have therapeutic potential by themselves or as adjuvant to conventional chemotherapy into clinical trial in near future.